I had the wrong function name prefix in the method_prefixes array: the
MAC functions all begin with ssh2_mac_* instead of ssh_mac_*. As a
result, MAC objects in the Python testcrypt system didn't provide
OO-like methods such as m.update() and m.genresult(); instead you had
to say ssh2_mac_update(m, ...) and ssh2_mac_genresult(m).